The importance of ultrashort pulse lasers for processing a wide range of materials is increasing more and more. In particular, ultrashort pulse laser radiation can be used to produce cuts in transparent materials (transparent for the radiation wavelength) deep in the material, i.e. below the surface. In the case of the short pulse durations in question here in the range of pico-femto- or attoseconds, the non-linear interaction processes between laser radiation and material that are responsible for the material separation (disruption) are concentrated mainly on the region of the beam focus, so that a high cutting accuracy can be achieved with a limited zone of undesirable collateral damage at the same time. In medical applications in particular, e.g. in laser-based eye surgery, the comparatively low energy density accompanying the ultrashort pulse durations that is required to achieve the desired interaction between radiation and tissue is a great advantage.
Ultrashort pulse lasers, especially those delivered as “turnkey” systems to customers by a laser manufacturer, are systems with a high degree of complexity, which usually require constant monitoring and adjustment of the pulse parameters to maintain the efficacy of the material processing at a desired level. In laser applications using ultrashort pulse radiation, the interaction between the radiation and the material is based primarily on non-linear processes, which call for a certain intensity and energy density. This can be achieved by strong focusing of the radiation pulses with the result that in the material to be processed, a very high peak intensity (e.g. more than 10 TW/cm2) and energy density (e.g. more than 500 J/cm3) can occur in an extremely small volume.
As soon as an optimal set of pulse parameters has been determined for a specific material to be processed, it is often necessary for uniform processing to maintain the pulse properties in a certain range (not too low and not too high), in order to keep the desired primary interaction effects and any undesirable, possibly harmful secondary effects in balance. Two approaches can generally be distinguished when monitoring the beam quality of a laser beam: the sequential method and the simultaneous method. In the sequential method, the test is carried out timewise before the actual material processing in a separate upstream test step, in which e.g. sample pieces are processed by way of testing or certain beam parameters are measured directly at the location of the beam focus. In the simultaneous method, on the other hand, the beam test takes place during the actual material processing. The two methods can be described as an offline method (sequential) and an inline method (simultaneous). In material processing using ultrashort pulse laser radiation, a peak intensity that is stable from pulse to pulse in the target region is a key parameter for reliable and effective processing. In applications of this kind, inline monitoring of the peak intensity is therefore desirable.
An inline method for monitoring the quality of a laser beam is known from WO 2011/060707 A1. In this, a portion of the laser beam used for the material processing is coupled out and focused on a non-linear crystal, in which the second harmonic of the basic wavelength of the laser beam is produced by non-resonant frequency mixing. The conversion efficiency of the frequency doubling is in a direct ratio to the peak intensity of the laser pulses applied to the crystal and consequently depends on the peak intensity of the pulses of the (main) laser beam. Power changes in the second harmonic can—insofar as other influence factors can be excluded—be traced back accordingly to a change in the beam quality and/or the pulse duration.